A hormone-like cell growth factor (Sarcoma Growth Factor - SGF) was detected, isolated, and characterized. SGF effected a reversible malignant phenotypic change in cultured cells. These studies were done to determine how these substances biochemically bring about the morphological phenotype of transformed cells. Serum-free conditioned media was searched for growth factors from MSV-transformed 3T3 cells. An acid soluble peptide extract stimulated 3H-thymidine incorporation into the DNA of serum depleted cells, completed with radiolabeled EGF for plasma membrane receptors, and stimulated anchorage independent growth of untransformed cells in soft agar. Comparison of SGF and EGF (Epidermal Growth Factor) showed that while they share some common properties they are distinctly different. Both interact with the EGF membrane receptor system, are heat stable peptides requiring disulfide bonds for activity, and are active in nanogram quantities. They differ in that SGF allows soft-agar growth where EGF will not; SGF is produced by MSV-transformed cells whereas EGF is not. Antibodies directed against EGF do not recognize SGF, and their isoelectric points are distinctly different, EGF being 4.4 and SGF being 6.8.